1. Field of the Invention
The present invention relates to a method of surface treatment of metallic materials, more particularly, to a method of nanocrystallizing the surface of metallic materials by bombarding of supersonic fine particles.
2. General Background and State of the Art
It is well known that material failures mostly occur on the surface of materials. Most of material failures, for example, such as fatigue fracture, fretting fatigue, wear, corrosion and the like, are very sensitive to the structure and properties of the material surface. Optimization of surface structures and properties may effectively improve combined properties of the material. As a result, the surface-modification of engineering materials is used in more and more industrial applications to greatly improve the behavior of materials. With the increasing of evidences of novel properties in nanocrystalline materials, it is necessary to provide a method for surface-modifying by the generation of a nanostructured surface layer, through which the combined properties and behavior of the materials can be significantly improved. This kind of surface modification, i.e., surface nanocrystallization (SNC), will greatly enhance the surface properties without changing the chemical composition. For example, the material which surface is nanocrystallized may have a strong and broad-spectrum absorption property. It is also a flexible method which is likely to fulfill specific requirements for structure/property of the surface of the sample. Surface nanocrystallization of materials can be carried out by using various processes. Among them, one is based on various coating and depositing technologies such as PVD, CVD and spraying methods. The coated materials can be either nanometer-sized isolated particles or polycrystalline powders with nano-sized grains. The coated layers and the matrix can be made of different or same materials. The predominant factor of this process is the bonding of the coated layer with the matrix. Another type of surface nanocrystallization is to transform the surface layers of the materials into nanocrystalline states while maintaining the overall composition and/or phases unchanged, and such a method is so-called surface self-nanocrystallization (SSNC).
Most of conventional mechanical surface treatment methods can be used for the SSNC. For example, ultrasonic shot peening (USSP) method has been used in surface self-nanocrystallization of metallic materials [J.Mater.Sic.Technol. 1999, Vol. 15(3): 193.]. In this paper, a concept of surface nanocrystallization (SNC) of metallic materials and surface self-nanocrystallization of metallic materials by ultrasonic shot peening was introduced. French patent No. 2689431 disclosed a method, in which, ultrasonic shot peening is carried out by the vibration of spherical shots (diameter thereof is 2 mm) created by high power ultrasound. The shots are placed in a reflecting chamber (including an ultrasonic concentrator) that is vibrated by a supersonic generator, after which the shots are resonated. Because of the high frequency of the system (20 kHz), the entire surface of the component to be treated is peened with a large number of impacts over a short period of time to obtain nanostructure layer. But this method is not suitable for the treatment of surface of complicated or larger parts.
Russian patent No. 1391135 disclosed a gas-dynamic spraying method for applying a coating. Upon which, the level of thermal stress can substantially reduced, and the thermal chemical action upon coated surface and powder particles can be weakened, and initial structure of the powder material can be preserved and there are no phase transformations, over-saturated structures or evaporation during application and formation of coatings. The aim of the patent only is to form an additional over-layer on the surface of materials but not to realize surface self-nanocrystallization of metallic materials.